During the operation of a diesel engine vehicle, it may be desirable to idle the engine for a prolonged period of time, for example to heat a vehicle cabin or run engine accessory devices. Diesel engines may emit NOx gases as exhaust, which are regulated by law. For example CARB 1956.8 (a) (6) mandates vehicles with gross vehicle weight rating (GVWR) greater than 14,000 lbs (6,350 kilograms) have emissions lower than 30 grams per hour while the engine is idling or the engine must shut down within 5 minutes of exceeding the limit.
Previous art has addressed the control of NOx emissions in various ways. Exhaust gas recirculation (EGR) systems and after-treatment devices may reduce emitted NOx. EGR systems may reduce emitted NOx by recirculating exhausted gases back to the engine's intake. Examples of NOx after-treatment devices include selective catalytic reduction (SCR) devices and three way catalysts (TWC). Treatment of exhaust gases by a catalyst may be carried out at or above a light-off temperature that provides the activation energy for the emission neutralizing reactions to take place. Vehicles may also employ automatic shut-down controls which turn an engine off after a predetermined time.
The inventors herein recognize difficulties with these approaches to emissions control during idling. An idling engine may produce exhaust gases that are cooler than exhaust gases produced at higher speeds and/loads. Cooler exhaust gas temperature may lead to coke formation, soot accumulation, or other contamination inside an EGR system, and may lower the temperature in after-treatment devices below light-off as well as facilitate the formation of urea deposits in a SCR device. Additionally, if an operator desires to run an engine in idle for longer than 5 minutes, then automatic shutdown controls may degrade customer satisfaction.